As a cathode structure of a plasma-arc torch in the prior art, a rod-shaped cathode structure and a buried type cathode structure are known as shown, for example, in "PRAZUMA SETSUDAN NO KISO TO JISSAI (Fundamental and Practice of Plasma-arch Cutting)" (in Japanene) edited by NIHON YOSETSU KAI (Society of Welding in Japan) (published Dec. 1, 1983 by KOSAIDO SANPOH SHUPPAN, page 48).
The rod-shaped cathode is a cathode mainly making use of inert gas such as argon, nitrogen, hydrogen, etc. as a working gas, and it is employed in a torch having a relatively small capacity, whereas the buried type cathode is a cathode wherein kafnium, zirconium or the like is buried in a tip end portion of a water-cooled copper pipe, and it mainly makes use of an oxidizing working gas such as oxygen, air, etc. and is employed in a torch having a relatively large capacity.
In addition, with regard to a cathode structure of non-transition type plasma-arc torch, in the case of large-capacity torches, cathodes having various shapes and structures have been devised. For instance, besides the above-mentioned rod-shaped or buried type cathodes, cathodes having a ring-shaped or hollow type (hallow cathode) structure have been used.
In common to the transition type and non-transition type plasma-arc torches, one big problem is abrupt consumption of a cathode and a nozzle accompanying the generation of plasma arcs. Especially in the case where oxidizing gases such as air, oxygen, etc. are used as a working gas, their life would become extremely short, and as a cathode, the life is so short that it becomes necessary to be replaced at an interval of about 0.5.about.3 hours.
Heretofore, because of the necessity of frequent cathode replacement works due to such a short period of life of the cathode, for instance, version to numerical control (NC) of a plasma-arc cutting machine and a plasma-arc welding machine as well as popularization of these machines are remarkably lagging.
Now, it has been well known that among the heretofore employed cathode structures and working gas feed systems in the transition type plasma-arc torches, there are an axial flow type and a swirl flow type. And in these plasma-arc torches of either axial flow type or swirl flow type in the prior art, a discharge point exists always at the center of the cathode, and as the time when a plasma arc is generated elapses, consumption of the cathode proceeds remarkably from its central portion.
As one method for resolving the problem of abrupt cathode consumption, it was devised by the inventor of this application to continuously move the discharge point on the cathode surface to a new position during generation of a plasma arc. The device is disclosed in Japanese Utility Model Application No. 60-130799 filed on Aug. 29, 1985. The cathode structure of the transition type plasma-arc torch based on this device is the structure shown in FIG. 1 of the accompanying drawings.
In FIG. 1, reference numeral 10 designates a cathode holder, and to the bottom end portion of this cathode holder 10 is mounted a cathode 11. The cathode 11 is provided with a hemispherical concave surface 12 on its bottom surface. In addition, the cathode holder 10 is provided with a generating device of lines of magnetic force 13 such as a coil, a permanent magnet or the like, and this generating device of lines of magnetic force 13 is disposed above the cathode 11 coaxially therewith. In the case where the generating device of lines of magnetic force 13 is a coil, a D.C. coil is used. Reference numeral 14 designates a nozzle for swirling a working gas, numeral 15 designates lines of magnetic force, numeral 16 designates lines of electric force, numeral 17 designates a discharge point, numeral 18 designates a plasma arc, and numeral 19 designates a nozzle.
Now description will be made on an operation of the plasma-arc torch shown in FIG. 1.
When the generating device of lines of magnetic force 13 operates, lines of magnetic force 15 as shown by dash lines are formed. In addition, since lines of the electric force 16 are formed in the perpendicular direction with respect to the bottom surface of the cathode 11, a vector product (E.times.B) of the both would take an effective value not equal to zero, this is, E.times.B.noteq.0 almost over the entire region of the cathode bottom surface except for the center axis (the axis of symmetry) on which the respective directions of the line of electric force vector E and the line of magnetic force vector B coincide with each other. It is expected that in the proximity of the discharge point 17 in the figure, the product value takes the maximum value.
Accordingly, a Lorentz force serving as a force for driving the discharge point 17 EQU F=j.times.B=.sigma.E.times.B
would similarly take the maximum value. It is to be noted that .sigma. represents a conductivity, and j represents a current density of a current flowing through the plasma. And since the array of the lines of magnetic force (the magnetic field arrangement) is symmetric with respect to an axis, a revolving motion of the discharge point 17 as shown in FIG. 1 is induced.
In the above-described cathode structure in a plasma-arc torch shown in FIG. 1, since the volume of cathode material becomes large, a cooling effect by coolant medium was poor. Especially, in the case where oxidizing gas such as oxygen, air or the like is used as a working gas, often hafnium, zirconium or the like is used as cathode material. As hafnium and zirconium are metals having a very small thermal conductivity, the known cathode structure had a shortcoming that when these metals are used as cathode material, if its volume is made large, the temperature in the proximity of the discharge point would rise, and an amount of local consumption of a cathode would become large.